A typical balance pressure valve comprises a pressure balanced shaft or plug with through holes to ensure that pressure forces are balanced on either side of the shaft or plug. The balance chamber over the plug may be a zone of zero fluid velocity where any particles carried suspended in a fluid to that zone may drop out of suspension there. Thus these through holes or balance holes can allow unwanted particulate material, e.g. pipe scale, grit and other debris, to pass through the plug. Such material can then cause sticking of the valve and possible damage to the valve. Although it is possible in principle to provide the balance holes with a conventional filter to prevent passage of particulates, should the filter become blocked, the balance holes will be unable to perform their necessary function of equalling pressure in the balance chamber, whether such change is due to fluid pressure changes or to volume displacement on opening and closing the valve.
It is an object of the present invention to provide a filter for a valve that overcomes these difficulties.